1. Field of the Invention
The present invention relates to the field of piezoelectric crystals and piezoelectric crystal composites operating for high frequency (>20 MHz). More particularly, the present invention provides high frequency piezoelectric crystal composites for high resolution imagery for preferred use in industrial and medical ultrasound applications, and even more particularly to the methods of manufacturing the same.
2. Description of the Related Art
Conventionally, PMN-PT based piezoelectric single crystals have superior dielectric and piezoelectric properties compared to the traditional PZT ceramics. To more fully exploit the excellent properties of single crystals, crystal composites have been fabricated to improve the electromechanical coupling coefficient and thus transducer performance characteristics.
For ultrasound transducers, the operating frequency is inversely related to the thickness of the piezoelectric material. Thus, as the targeted operating frequency increases, the thickness of piezoelectric material decreases accordingly this induces operative and electro mechanical difficulties. On the other hand, an optimal aspect ratio has been attempted for the piezoelectric crystal pillars in order to maintain the high electromechanical coupling coefficient of piezoelectric composite. To accommodate the requirements in thickness and aspect ratio, the feature size of the piezoelectric material in the high frequency composite needs to be reduced to meet the optimal ratio.
One attempt has been provided for such medical applications of micromachined imaging transducers known generally from U.S. Pat. No. 7,622,853 (Rehrig et al., assigned to SciMed Life Systems, Inc.), the entire contents of which are incorporated herein by reference.
As noted in U.S. Pat. No. 7,622,853, a medial device is provided with a transducer assembly including a piezoelectric composite plate formed using photolithography micromachining. The particular steps in the '853 patent are noted therein. The '853 patent additionally notes the conventional challenges of micromachining poled PZT ceramics, but fails to adjust to the now appreciated challenges noted below and additionally includes the detrimental impacts of electric field and clamping effect on strain. There is now appreciated a need for further imagery resolution and sensitivity over a depth that cannot be achieved.
Finally, it is further recognized that a high frequency transducer is typically driven at a higher electrical field compared to a low frequency transducer.
Accordingly, there is a need for an improved high frequency piezoelectric crystal composite, optionally related devices, and further optionally methods for manufacturing the same.
Related publications include the following, the entire contents of each of which are incorporated herein fully by reference:    1. P. Han, W. Yan, J. Tian, X, Huang, H. Pan, “Cut directions for the optimization of piezoelectric coefficients of PMN-PT ferroelectric crystals”, Applied Physics Letters, volume 86, Number 5 (2005).    2. S. Wang, et al., “Deep Reactive Ion Etching of Lead Zirconate Titanate Using Sulfur hexafluoride Gas”, J. Am. Ceram. Soc., 82(5) 1339-1341, 1999.    3. A. M. Efremov, et al., “Etching Mechanism of Pb(Zr, Ti)O3 Thin Films in Cl2/Ar Plasma”, Plasma Chemistry and Plasma Processing 2(1), pp. 13-29, March 2004.    4. S. Subasinghe, A. Goyal, S. Tadigadapa, “High aspect ratio plasma etching of bulk Lead Zirconate Titanate”, in Proc. SPIE—Int. Soc. Opt. Engr, edited by Mary-Ann Maher, Harold D. Stewart, and Jung-Chih Chiao (San Jose, Calif., 2006), pp. 61090D1-9.